Inertial navigation of a platform is based upon the integration of specific forces and angular rates measured by inertial sensors (e.g. accelerometer, gyroscopes) by a device containing the sensors. In general, the device is positioned within the platform and commonly tethered to the platform. Such measurements from the device may be used to determine the position, velocity and attitude of the device and/or the platform.
The platform may be a motion-capable platform that may be temporarily stationary. Some of the examples of the platforms may be a vehicle or a vessel of any type. The vessel may be land-based, marine or airborne.
Alignment of the inertial sensors within the platform (and with the platform's forward, transversal and vertical axis) is critical for inertial navigation. If the inertial sensors, such as accelerometers and gyroscopes are not exactly aligned with the platform, the positions and attitude calculated using the readings of the inertial sensors will not be representative of the platform. Fixing the inertial sensors within the platform is thus a requirement for navigation systems that provide high accuracy navigation solutions.
For tethered systems, one known means for ensuring optimal navigation solutions is to utilize careful manual mounting of the inertial sensors within the platform. However, portable navigation devices (or navigation-capable devices) are able to move whether constrained or unconstrained within the platform (such as for example a vessel or vehicle), so careful mounting is not an option.
Existing portable navigation devices (or navigation-capable devices) cannot achieve accurate attitude and position of the platform unless at least one of the following three conditions is known:
1) absolute attitude angles for the device and the platform;
2) absolute attitude angles for the device and the misalignment between the device and platform; or
3) absolute attitude angles for the platform and the misalignment between the device and platform.
The first above option needs two assembly of sensors at least, one on the device and one on the platform, thus knowledge of misalignment is a key factor to enable portable navigation devices without the previously mentioned constraint.
For navigation, mobile/smart phones are becoming very popular as they come equipped with Assisted Global Positioning System (AGPS) chipsets (in addition to significantly improving the startup performance by utilizing network connection) also further use high sensitivity capabilities to provide absolute positions of the platform even in some environments that cannot guarantee clear line of sight to satellite signals. Deep indoor or challenging outdoor navigation or localization incorporates cell tower ID or, if possible, cell towers trilateration for a position fix when an AGPS solution is unavailable. Despite these two positioning methods that already come in many mobile devices, accurate indoor localization still presents a challenge and fails to satisfy the accuracy demands of today's location based services (LBS). Additionally, these methods may only provide the absolute heading of the platform without any information on the device's heading.
Many mobile devices, such as mobile phones, are equipped with Micro Electro Mechanical System (MEMS) sensors that are used predominantly for screen control and entertainment applications. These sensors have not been broadly used to date for navigation purposes due to very high noise, large random drift rates, and frequently changing orientations with respect to the carrying platform.
Magnetometers are also found within many mobile devices. In some cases, it has been shown that a navigation solution using accelerometers and magnetometers may be possible if the user is careful enough to keep the device in a specific orientation with respect to their body, such as when held carefully in front of the user after calibrating the magnetometer.
As such, there is a need for a navigation solution capable of accurately utilizing measurements from a device within a platform to determine the navigation state of the device/platform without any constraints on the platform (i.e. in indoor or outdoor environments) or the mobility of the device. The estimation of the position and attitude of the platform has to be independent of the usage of the device (e.g. the way the device is put or moving within the platform during navigation). In the above scenarios, the device can be moved or tilted to any orientation within the platform; it is required that the device provide seamless navigation even in such scenarios. This again highlights the key importance of misalignment determination between the device and platform, to enable the device to be used in any orientation with respect to the platform.
Thus methods of mitigating such problems are required for navigation using devices, wherein mobility of the device may be constrained or unconstrained within the platform, and wherein the device may be moved or tilted to any orientation within the platform.
In addition to the above mentioned application of portable devices (that involves a full navigation solution including position, velocity and attitude, or position and attitude), there are other applications (that may include estimating a full navigation solution, or an attitude only solution or an attitude and velocity solution) where the method to mitigate the aforementioned problems is needed for enhancing the user experience and usability, and may be applicable in a number of scenarios.